Field of the Invention
The invention relates to a circuit configuration for stabilizing the operating point of a transistor. Such circuit configurations are known, for example, from a publication entitled: Elemente der angewandten Elektronik [Elements of Applied Electronics], by E. Bxc3x6hmer, 9th edition, Vieweg-Verlag, Braunschweig, Germany, 1994 and use parallel degenerative feedback or series degenerative feedback.
In parallel degenerative feedback, a resistor is connected in parallel with the collector-base junction of the transistor to be controlled. An increase in the operating current, for example a temperature-induced increase, brings about a drop in the collector-emitter voltage and at the same time a drop in the base current. That produces a degenerative feedback effect.
In series degenerative feedback, a resistor is connected into the emitter lead. The base potential is set through the use of a voltage divider. In the case of a temperature-induced increase in the operating current, the emitter potential is raised, so that the base-emitter voltage, and consequently in turn the base current, is lowered. That in turn produces a degenerative feedback effect.
Apart from temperature fluctuations, fluctuations in the supply voltage and deviations from the nominal value of the current gain of the transistor to be controlled, caused by unavoidable production tolerances, have the effect of causing the operating current to depart from the desired value.
Fluctuations in the supply voltage are unavoidable, particularly in the case of battery-operated electronic devices. In view of the increasing proliferation of mobile electronic devices, such as mobile phones or laptop computers, battery operation is gaining increasingly in significance. Therefore, fluctuations in the supply voltage must be taken into account to a greater extent when constructing electronic circuits. In addition, mobile electronic devices are generally subject to great fluctuations in ambient temperature.
Both in the case of fluctuations in the supply voltage and in the case of deviations of the current gain from the nominal value, the frequently used parallel degenerative feedback only ensures inadequate stabilization of the operating current. In both cases, significant changes in the operating current occur.
Better stabilizing of the operating current in the case of a fluctuating supply voltage or in the case of a deviation of the current gain from the nominal value, can be accomplished with series degenerative feedback. However, it has the major disadvantage of there being a drop in voltage at the resistor connected in the emitter lead, and that voltage has to be additionally supplied by the supply voltage source. However, in view of the development prevailing in the case of electronic devices toward ever lower supply voltages, that is not advantageous.
German Patent DE 195 05 269 C1 discloses a circuit configuration which allows the operating point to be stabilized even in the case of relatively low supply voltages. According to that patent, the operating current is stabilized by connecting, between the collector and the base of the transistor to be controlled, a current-determining mesh including a series connection of at least two diodes, a resistor and the emitter-base junction of a further transistor. For many applications, however, the voltage drop occurring in that circuit configuration is still regarded as too high.
It is accordingly an object of the invention to provide a circuit configuration for operating point stabilization of a transistor, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which ensures an operating current of a transistor even in the case of a low supply voltage.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for stabilizing the operating point of a first transistor with an emitter, a base and a collector, the emitter of which is connected to a first supply potential. Preferably, a second transistor has an emitter, a base and a collector, the collector thereof is coupled to the base of the first transistor and the emitter thereof is coupled to the collector of the first transistor. A first resistor is connected between a second supply potential on one hand and the collector of the first transistor and the emitter of the second transistor on the other hand. A current-limiting element is connected between the base of the second transistor and the first supply potential. A third transistor has an emitter, a base and a collector, the emitter is connected to the second supply potential and the collector is connected to the base of the second transistor. A second resistor is connected between a reference potential and the base of the third transistor. A third resistor is connected between the base and the collector of the third transistor.
It is particularly the third transistor, in connection with the second and third resistors and the reference potential, which has the effect of permitting the voltage to firstly be set and secondly be set in such a way that the voltage drop across the first resistor is as small as possible. If, on the other hand, two diodes were used, one of the diodes would compensate for the voltage drop across the base-emitter junction of the second transistor and the further diode would then fix the voltage drop across the first resistor, namely to the forward voltage of the further diode. In the case of two diodes, the voltage drop across the first resistor would accordingly be approximately 700 mV, while in the case of the device according to the invention the voltage drop across the first resistor can be set, for example, to 100 mV and below. This brings a saving of 0.6 V and more which, for example in the case of customary supply voltages of 2.7 V, still amounts to a saving of over 22%.
In accordance with another feature of the invention, the base of the first transistor is coupled to the collector of the second transistor through a fourth transistor having an emitter, a base and a collector. This is done in such a way that the base of the fourth transistor is connected to the collector of the second transistor, the emitter of the fourth transistor is connected to the base of the first transistor and the collector of the fourth transistor is connected to the second supply potential.
An additional current amplifier formed by the fourth transistor allows the dependence of the stabilized current on the current gain of the first transistor to be considerably reduced. With the low voltage across the first resistor, which is evaluated for setting the operating point, the influence of the current gain of the first resistor is distinctly more disruptive than in the case of a higher voltage drop across the first resistor. Consequently, the application range (maximum collector current), which particularly in the case of bipolar high-frequency transistors is limited by a finite current gain, can also be extended, because a higher base current is possible. In addition, the temperature characteristic of the entire configuration can be advantageously changed greatly, to the extent of being inverted.
In accordance with a further feature of the invention, the temperature characteristic can advantageously be set by a further resistor in such a way that a temperature compensation occurs. For this purpose, a fourth resistor is connected between the base and the emitter of the fourth transistor.
In accordance with an added feature of the invention, the reference potential can be tapped at the tap of a voltage divider having a fifth and a sixth resistor, connected between the first supply potential and a third supply potential.
In accordance with still another feature of the invention, the third supply potential may be equal to the second supply potential or equal to the emitter potential of the second transistor.
In accordance with an additional feature of the invention, in the simplest case, the current-limiting element is formed by a resistor. Consequently, the circuit-related expenditure is kept extremely low. Furthermore, a current source may alternatively be provided. This allows the voltage dependence of the current consumption of the circuit configuration with respect to operating voltage fluctuations to be improved.
In accordance with yet another feature of the invention, the current source includes a fifth transistor with an emitter, a base and a collector, and the collector thereof is connected to the first supply potential. A seventh resistor is connected between the base of the second transistor and the emitter of the fifth transistor. A sixth transistor with an emitter, a base and a collector, has the emitter connected to the second supply potential and the collector connected to the base of the fifth transistor. An eighth resistor is connected between the collector of the sixth transistor and the first supply potential. A ninth resistor is connected between the emitter of the fifth transistor and the base of the sixth transistor. A tenth resistor is connected between the base and the emitter of the sixth transistor.
In accordance with a concomitant feature of the invention, the first and fourth transistors are of a first conduction type and the second, third, fifth and sixth transistors are of a second conduction type.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for operating point stabilization of a transistor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.